The passengers were mostly foreigners from construction company Nass-Murray and Roberts who were celebrating completion of part of the Bahrain World Trade Centre towers.

Unofficial tallies suggested that 25 Britons, 20 Filipinos, 10 South Africans and 10 Egyptians were among those on board.

The UK's acting ambassador in Bahrain, Steve Harrison, told the BBC it was difficult to confirm how many Britons had died as there was no definitive passenger list and identification was proving difficult.

"It was a party and quite a few of those on board were not carrying identification," he said.

'Too little time'

Bahrain's coastguard chief Yussef al-Ghatim told AFP news agency "the organiser of the trip said 150 people were invited but some 14 to 20 may have gotten off before it sailed".

The boat capsized at about 2145 local time (1845 GMT), near one of the bridges linking the capital Manama with al-Muharraq island, Mr al-Ghatim said.

An Indian barman who had been onboard told the Gulf Daily News that the boat had "rocked badly and tilted over" after hitting a wave.

"The refrigerator and cooker all slid to one side of the boat and with that extra weight, the boat couldn't sustain it any longer and we rolled," he said.

The paper also said a South African man had warned the passengers not to gather on the dhow's upper deck before it set out.

"He got the microphone and told them to go down because the boat was rocking from side to side," a witness said. "There were life jackets on board, but the boat toppled very fast - there wasn't enough time."

Little warning

Eyewitnesses told the BBC how the al-Dara dramatically capsized in apparently calm waters as it made a turn.

We saw [the ship] leaning on its side quickly then it capsized and its lights turned off

Abd Ali Muhammad Hussan told the BBC Arabic service the boat had quickly got into trouble.

"We saw it leaning on its side quickly then it capsized and its lights turned off.

"We moved backward towards them and managed to save seven of the crew... The people we saved were foreign tourists, not Bahrainis," Mr Hassan said.

The incident happened less than a mile off the coast, Bahraini and US officials said. The harbour waters are generally shallow but with some deep channels.

'Overcrowded'

Correspondents and eyewitnesses reported the boat appeared heavily overcrowded before it set off into the harbour.

Rescue workers and volunteers pulled dozens of bodies ashore

Bahrain's coastguard service launched an immediate rescue operation, reaching the scene within minutes.

US divers and small naval craft from the US 5th Fleet also joined the rescue efforts.

"We were nearby - it happened about one mile east of where we're headquartered so as soon as we got the call we immediately started to move our people and our boats to that area within probably about 15 or 20 minutes," Commander Jeff Breslau, spokesman for the US 5th Fleet, told the BBC.

"We provided a US Coastguard cutter, along with 16 divers, one helicopter in the air and another on standby in order to support whatever the Bahrainis needed for the effort," he said.